1. Field of the Invention
The present invention relates to a mobile (radio or wireless) communication system and, more particularly, to a signal transmitting and receiving method in a closed-loop space-time transmit diversity (STTD) system having a plurality of transmission antennas.
2. Description of the Background Art
In general, in a mobile (radio or wireless) communication system, when data is transmitted at high speeds, the phase of a received signal is distorted due to a fading phenomenon (i.e., signal weakening) generated on a wireless channel. Fading reduces the amplitude of a received signal from a few decibels (dB) to scores of dB. Thus, if the distorted phase of the received signal is not compensated during data demodulation at the receiving end, information errors occur in the received data, and the quality of the overall mobile communication service is undesirably degraded.
Thus, in order to transmit data at high speeds without degradation of service quality, various types of signal transmission diversity techniques are employed,. An example is disclosed in, S. M. Alamouti, “A Simple Transmit Diversity Technique for Wireless Communications”, IEEE Journal on Select Areas in Communications, Vol. 16, No. 8, pp. 1451-1458, October 1998. Also, additional background information can be found in 3GPP, “Physical Channels and Mapping of Transport Channels onto Physical Channels (FDD)”, TS 25,211, V4.3.0, December 2001.
Transmission diversity techniques are generally classified into time diversity and space diversity. Time diversity, which uses interleaving and coding to overcome the fading, is commonly used for a Doppler spread channel. However, time diversity is not suitable for use in a low speed Doppler spread channel.
Space diversity is used for channels with a small delay spread (such as a so-called “indoor channel” used for indoor environments) and channels with a low speed Doppler spread (such as a so-called “pedestrian channel” used for pedestrians). Space diversity is a transmission diversity technique using two or more transmission/reception antennas. That is, upon transmitting the same signal through two transmission antennas, if the receiving end determines that the size of the signal transmitted through one antenna has been undesirably reduced due to fading (i.e., signal weakening), the signal transmitted through the other antenna is selected and received by the receiving end.
Space diversity is further classified into a reception antenna diversity (using multiple reception antennas), and a transmission antenna diversity (using multiple transmission antennas). For transmission antenna diversity, an algorithm used for receiving downlink signals and obtaining a diversity gain can be roughly divided into an open-loop mode and a closed-loop mode.
The third-generation partnership project (3GPP) currently employs a space-time transmit diversity (STTD) technique, which is a type of open-loop mode transmission antenna diversity. The STTD is a technique for obtaining a diversity gain through space-time coding, which is based upon the channel coding technique commonly applied on a time axis framework, but being extended into a spatial domain. Namely, the STTD is used to obtain a space diversity gain as well as a time diversity gain by performing coding among the symbols transmitted respectively through each antenna when two transmission antennas are used.
The STTD can be applied to every type of downlink physical channel, except a synchronization channel used in WCDMA. Open-loop STTD is advantageous in that it causes no detrimental changes in radio communication performance due to varying data transmission speeds, because a feedback signal is not necessary.
The general operation principles of the STTD technique for performing coding among the data symbols transmitted through two antennas will now be described.
The STTD encoding for two antennas and a transmission order based on time are shown in Table 1.
TABLE 1Time (t)Time (t + T)Antenna 1s1s2 Antenna 2−s2*s1*wherein, ‘s’ is a data symbol, and ‘T’ is a data symbol period.
Referring to Table 1, the symbols (s1,s2) to be transmitted are encoded by an STTD encoder and then respectively sent to the two transmission antennas (antennas 1 and 2) in time consecutive order. That is, for antenna 1, symbol s1 is sent at time t, and symbol s2 is sent at a later time, t+T. For antenna 2, symbol −s2* is sent at time t, and symbol s1* is sent at a later time, t+T.
In other words, the STTD encoder outputs s1 and s2 to transmission antenna 1 as they are (i.e., without any further changes thereto after encoding), but the STTD encoder converts the symbols s1 and s2 into −s2* and s1*, respectively, and then outputs them to antenna 2. Here, “*” indicates a conjugate. The symbols (s1, s2, −s2*, s1*) sent to the transmission antennas are then transmitted to a receiving end via a multipath manner.
Assuming that the data symbols transmitted through each transmission antenna pass through respectively different independent channels, and assuming that there are no variations in the channel at time (t) and at time (t+T), the received signals (r1 and r2) at the receiving end can be defined by equations (1):r1=r(t)=h1s1−h2s2*+n1r2=t(t+T)=h1s2+h2s1*+n2  (1)wherein h1(=α1ejβ1) and h2(=α2ejβ2) represent the channels (e.g., are channel responses) between one transmission antenna (either antenna 1 or antenna 2) and the reception antenna, respectively, while n1 and n2 are noise factors referred to as an “additive white Gaussian noise” (AWGN), which is a complex Gaussian noise. Also β1 and β2 are phases of fading channels h1 and h2, respectively. Additionally α1 and α2 are amplitudes of fading channels h1 and h2, respectively
Each channel (h1 and h2) can be estimated by using a pilot signal transmitted from each transmission antenna. Also, if the received signals (r1 and r2) are combined together in a manner shown in equations (2) below, the resulting output values (estimate (or deduced) symbols (ŝ1,ŝ2)) are equivalent to the output values obtained when using a maximum ratio combining (MRC) method of reception diversity, and thus the received symbols can be properly deduced (estimated).ŝ1=h1*r1+h2r2*=(α12+α22)s1+h1*n1+h2n2*ŝ2=h1*r2−h2r1*=(α12+α22)s2+h1n2*+h2*n1  (2)1) Related Art 4-Antenna Open-Loop STTD System
FIG. 1 is a drawing illustrating an open-loop STTD transmitting end using four antennas in the related art.
As shown in FIG. 1, the 4-antenna open-loop STTD transmitting end includes an STTD encoder 10 for performing space-time coding on data symbols to be transmitted; first multipliers 11 and 12 for multiplying certain gain factors (χ and ξ) to the data symbols outputted from the STTD encoder 10; second multipliers 13 and 14 for multiplying certain phase rotations (θ1 and θ2) to the outputs of the first multipliers 11 and 12; and four transmission antennas (A1-A4) for transmitting the outputs of each multiplier 11 through 14.
As shown in FIG. 1, upon receiving original data symbols to be transmitted, the STTD encoder 10 performs encoding and generates first data symbols (s1, s2, s3 and s4). The STTD encoder 10 then sends the first data symbols as they are (i.e., without any further changes thereto after encoding) for further processing and ultimately to the antennas A1 and A2.
On the other hand, the STTD encoder 10 performs encoding and processing to generate second data symbols (−s2*, s1*, −s4* and s3*). Here, the processing includes converting the first data symbols (s1, s2, s3 and s4) into conjugate data symbols (−s2*, s1*, −s4* and s3*), respectively. Thereafter, the second data symbols (−s2*, s1*, −s4* and s3*) are output for further processing and ultimately sent to the antennas A3 and A4. Here, ‘*’ signifies a conjugate.
The first data symbols (s1, s2, s3 and s4) outputted from the STTD encoder 10 are multiplied by a predetermined gain (χ) in the multiplier 11, and delayed by a predetermined phase (θ1) in the multiplier 13. Meanwhile, the second data symbols (−s2*, s1, −s4* and s3*) outputted from the STTD encoder 10 are multiplied by a predetermined gain (ξ) in the multiplier 12, and then delayed by a predetermined phase (θ2) in the multiplier 14. Accordingly, the transmission antennas (A2 and A4) transmit the first data symbols with phase differences of θ1 and θ2, respectively, compared to the second data symbols transmitted by transmission antennas A1 and A3. Here, the gains (χ and ξ) are each assumed to be ‘1’ merely to simplify calculations,
Accordingly, signals containing data symbols transmitted through each transmission antenna (A1˜A4) for each symbol duration (T) can be expressed by equation (3):
                              [                                                                      A                                      1                                                                                                                                             A                                      2                                                                                                                                             A                                                            3                      1                                                                                                                                                                 A                  4                                                              ]                =                  [                                                                      s                  1                                                                              s                  2                                                                              s                  3                                                                              s                  4                                                                                                                          ⅇ                                          j                      ⁢                                                                                          ⁢                      θ1                                                        ⁢                                      s                    1                                                                                                                    ⅇ                                          j                      ⁢                                                                                          ⁢                      θ1                                                        ⁢                                      s                    2                                                                                                                    ⅇ                                          j                      ⁢                                                                                          ⁢                      θ1                                                        ⁢                                      s                    3                                                                                                                    ⅇ                                          j                      ⁢                                                                                          ⁢                      θ1                                                        ⁢                                      s                    4                                                                                                                        -                                      s                    2                    *                                                                                                s                  1                  *                                                                              -                                      s                    4                    *                                                                                                s                  3                  *                                                                                                                          -                                          ⅇ                                              j                        ⁢                                                                                                  ⁢                        θ2                                                                              ⁢                                      s                    2                    *                                                                                                                    ⅇ                                          j                      ⁢                                                                                          ⁢                      θ2                                                        ⁢                                      s                    1                    *                                                                                                                    -                                          ⅇ                                              j                        ⁢                                                                                                  ⁢                        θ2                                                                              ⁢                                      s                    4                    *                                                                                                                    ⅇ                                          j                      ⁢                                                                                          ⁢                      θ2                                                        ⁢                                      s                    3                    *                                                                                ]                                    (        3        )            
The signal received during four symbol durations (4T) at the receiving end having one reception antenna can be expressed as equations (4):r1=(h1+h2ejθ1)s1−(h3+h4ejθ2)s2*+n1r2=(h1+h2ejθ1)s2+(h3+h4ejθ2)s1*+n2r3=(h1+h2ejθ1)s3−(h3+h4ejθ2)s4*+n3r4=(h1+h2ejθ1)s4+(h3+h4ejθ2)s3*+n4  (4)
If h1+h2ejθ1 and h3+h4ejθ2 are substituted by ‘a’ and ‘b’, respectively, the signals received during the four symbol durations can be simplified to equations (5):r1=as1−bs2*+n1r2=as2+bs1*+n2r3=as3−bs4*+n3r4=as4+bs3*+n4  (5)
Using equations (5), the original data symbols transmitted from the transmitting end can be deduced at the receiving end (i.e., can calculate an estimate of the original data symbols) by using equations (6):ŝ1=a*r1+br2*=(a2+b2)s1+a*n1+bn2*ŝ2=a*r2−br1*=(a2+b2)s2+a*n2−bn1*ŝ3=a*r3+br4*=(a2+b2)s3+a*n3+bn4*ŝ4=a*r4−br3*=(a2+b2)s4+a*n4−bn'*  (6)
The conventional 4-antenna open-loop STRD transmission method explained above shows an excellent performance when a terminal moves at a high speed, i.e., a fast-moving terminal (e.g., the user is in a moving vehicle), but the performance is degraded if the terminal moves at a low speed, i.e., a slow-moving terminal (e.g., the user is walking).
Here, the terms “high speed” and “low speed” are relative expressions, but may be defined as required by the radio communication environment. For example, a threshold speed for a moving terminal may be set to be 10 km/h (kilometers per hour). So, any movement at or over this threshold speed would be considered as “high,” while any speed below the threshold would be considered as “low.” That is, if the terminal is slow-moving, severe fading (i.e., signal weakening) may occur on the transmission path (e.g., a channel) of a specific transmission antenna used for the slow-moving terminal. Thus, If a transmission signal is lost due to severe fading at a specific antenna, transmission power must be undesirably further consumed to re-transmit the signal to the terminal.
Therefore, if the terminal moves at a low speed, a closed-loop STTD transmission method that employs feedback is preferably used to maximize the gain of the transmission antenna diversity. In other words, by using closed-loop STTD for slow-moving terminals, a better performance (e.g., signal connectivity) than that of the open-loop STTD transmission method can be obtained, because the closed-loop STTD transmission method uses the reception information of each antenna provided from the terminal (i.e., feedback is provided). Thus, by using both the open-loop and closed-loop STTD techniques, optimal performance (e.g., signal connectivity) can be obtained whether the terminal is fast moving or slow moving.
2) Related Art 2-antenna Closed-loop STTD System
FIG. 2 is a drawing illustrating a 2-antenna closed-loop STTD system using two transmission antennas in accordance with the related art. As shown in FIG. 2, the 2-antenna closed-loop STTD system includes a STTD transmitting end having two transmission antennas (Tx1, Tx2) and a STTD receiving end having one reception antenna (Rx).
The STTD transmitting end can include: an STTD encoder 20 for performing space-time coding on data symbols to be transmitted; multipliers 21 and 22 for multiplying predetermined weight values w1 and w2 to the data symbols outputted from the STTD encoder 20; and two transmission antennas Tx1 and Tx2 for respectively transmitting the outputs of the multipliers 21 and 22.
The STTD receiving end can include: one receiving antenna (Rx1); a STTD decoder 23 for performing space-time decoding on a signal received via the receiving antenna (Rx1); a cross-interference conversion unit 24 for processing output signals (d1, d2) of the STTD decoder 23 to generate estimate symbols (ŝ1, ŝ2); and a weight calculator 25 for calculating the weight values (w1, w2) and feeding back the information to the STTD sending end.
The signal transmission method in the related art 2-antenna closed-loop STTD system will now be described.
Upon receiving original data symbols to be transmitted, the STTD encoder 20 in the transmitting end performs encoding and generates first data symbols (s1, s2), which are sent for further processing and ultimately to the first antenna Tx1.
On the other hand, the STTD encoder 20 performs encoding and processing to generate second data symbols (−s2*, s1*). Here, the processing includes converting the first data symbols (s1,s2) into conjugate data symbols (−s2*, s1*), respectively. Thereafter, the second data symbols (−s2*, s1*) are sent for further processing and ultimately to the second antenna Tx2. Here, ‘*’ signifies a conjugate.
The first data symbols (s1, s2) outputted from the STTD encoder 20 are multiplied by the weight value (w1) in the multiplier 21 and then sent to the first antenna Tx1, while the second data symbols (−s2*, s1*) are multiplied by the weight value (w2) in the multiplier 22 and then sent to the second antenna Tx2.
At the STTD receiving end having one reception antenna, the signals received during two symbol durations (2T) can be expressed as equations (7):r1=w1h1s1−w2h2s2*+n1r2=w1h1s2+w2h2s1*+n2  (7)wherein h1(=αejβ1) and h2(=α2ejβ2 ) indicate the channels (i.e., channel responses) between the transmission antennas Tx1 and Tx2, and the reception antenna Rx, respectively, while n1 and n2 indicate an additive white Gaussian Noise (AWGN).
The STTD decoder 23 of the STTD receiving end performs space-time decoding on the received signals (r1, r2) received through the reception antenna Rx, and then generates and outputs decoded signals (d1,d2) expressed as equations (8):d1=h1*r1+h2r2*=(w1|h1|2+w2|h2|2)s1+(w1−w2)h1*h2s2*+(h1*n1+h2n2*)d2=h1*r2−h2r1*=(w1|h1|2+w2|h2|2)s2+(w2−w1)h1*h2s1*+h1*n2−h2n1*)  (8)Assuming that (w1|h1|2+w2|h2|2)=A, (w1−w2)h1*h2=B, (h1*n1+h2n2*)=C1 and (h1*n2−h2n1*)=C2, the above equations (8) can be simplified to equations (9):d1=AS1+BS2*+C1d2=AS2−BS1*+C2  (9)
The cross interference conversion unit 24 processes the decoded signals (d1,d2) outputted from the STTD decoder 23, and generates estimate symbols (ŝ1,ŝ2) that estimate (deduce) the original symbols transmitted by the transmitting end. That is, in order for the receiving end to estimate (deduce) the original symbols, the cross interference conversion unit 24 performs signal processing by using equations (10):ŝ1=A*d1−Bd*2=(|A|2+|B|2)s1+(A*C1−BC2*)ŝ2=A*d2+Bd1*=(|A|2+−B|2)s2+(A*C2+BC1*)  (10)
Meanwhile, the weight calculator 25 calculates the weight values (w1, w2) from the received signals (r1, r2) received via the reception antenna (Rx1), and feeds back the weight values (w1, w2) to the multipliers 21 and 22 of the STTD transmitting end. Here, the weight calculator 25 calculates a weight vector that maximizes the value A in the equation, (w1|h1|2+w2|h2|2)=A. The reason for doing so is because, as shown in equations (10), the value A has the greatest influence in determining the “power of each symbol,” that is, the transmission power required to transmit each data symbol. Namely, based upon the characteristics of w12+w22=1 and dA/dw2=0, the weight calculator 25 calculates each weight value (w1, w2) as shown in equations (11):
                                          w            1                    =                                                                                      h                  1                                                            2                                                                                                                                      h                      1                                                                            4                                +                                                                                                h                      2                                                                            4                                                                    ,                              w            2                    =                                                                                      h                  2                                                            2                                                                                                                                      h                      1                                                                            4                                +                                                                                                h                      2                                                                            4                                                                                        (        11        )            
Referring to the above, the present inventors have recognized a problem in the related art method. Namely, the weight values calculated by the weight calculator 25 have not been induced from an optimal weight vector that maximizes the estimated symbol transmission power. In other words, in order to maximize the transmission power of symbols (ŝ1,ŝ2) that are estimated (deduced) at the receiving end, a weight vector that maximizes |A|2+|B|2 should be preferably calculated, in principle. However, because the related art weight calculation method generates weight values by inducing a weight vector that only maximizes A (and not |A|2+|B|2), the related art has shortcomings in that the transmission power of the symbols (ŝ1,ŝ2) estimated at the receiving end cannot be maximized.
By employing transmission antenna diversity when two or more transmission antennas are used, not only is a diversity gain obtained by the multiple-antenna transmission unit, but also a signal-to-noise ratio gain is also obtained. Thus, the signal-to-noise ratio gain increases proportionally to the number of transmission antennas used.
As shown in FIG. 2, current UMTS systems generally employ transmission diversity techniques for two transmission antenna systems. However, this method is restricted to situations where there is an STTD receiving end that performs transmission diversity for two transmission antennas Thus, the STTD receiving end operating under the current standards applicable to two transmission antenna systems would not operate properly if more than two transmission antennas are to be used.
In other words, the related art 2-antenna open-loop STTD system and method (e.g., FIG. 2) cannot be directly applied to closed-loop STTD systems employing more than two transmission antennas. For example, if four transmission antennas are to be used while still employing the related art method for transmitting signals with two antennas, the structures of the STTD transmitting end and the STTD receiving end of the closed-loop STTD system need to be changed, and the transmitting/receiving methods also need to be changed.
Similarly, the related art 4-antenna open-loop STTD technique (e.g., FIG. 1) cannot be applied to a 4-antenna closed-loop STTD technique, or at least there would be many difficulties in attempting to make such application.
Therefore, the present invention provides a structure and method for a closed-loop STTD being designed to be suitable for performing transmission antenna diversity when multiple (e.g., two or more) transmission antennas are employed.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.